In the oil and gas industry it is often necessary to deploy and operate a pump downhole in order to assist with hydrocarbon production from a well.
The hydrocarbons from such wells may often be produced in the form of a multiphase fluid, e.g. a fluid comprising one or more liquids such as water and/or crude oil and one or more gases such as natural gas.
Accordingly, it is preferred that a pump that is to be used downhole should be able to: (i) reliably handle multiphase fluids; (ii) generate sufficient pressure to lift fluids from deep hydrocarbon-bearing formations to the surface; and (iii) withstand and operate reliably in harsh downhole environments.
In order to generate sufficient pressure to lift fluids from deep hydrocarbon-bearing formations to the surface, it is known to use multistage pumps, i.e. pumps or pump assemblies containing a plurality of pump stages or modules, in which, typically, a first pump stage discharges into the intake of a second pump stage, which in turn discharges into a third pump stage and so on.
If a single pump stage is capable of generating a given differential pressure, say x psi, at a given flow rate, say y liters/hour, then a pump having two pump stages arranged in series could be constructed, which would be capable of generating a differential pressure of 2x psi at the flow rate of y liters/hour. If the two pump stages were arranged in parallel, then the pump would be capable of generating a differential pressure of x psi at a flow rate of 2y liters/hour.
It is known for oil well electric submersible pumps to use this principle to generate extremely high differential pressures, e.g. 2000-3000 psi (13.8-20.7 MPa). Such pumps may contain 100 or more pump stages arranged in series.
It is known to use multistage centrifugal pumps to lift fluids from deep hydrocarbon-bearing formations to the surface. Centrifugal pumps work by repeatedly accelerating and decelerating the fluid to add incremental pressure increases to the pumped fluid. When being used to pump a mixed-phase fluid containing a liquid and a gas, as a result of the density contrast between liquid and gas the liquid is preferentially accelerated in the first stages of a centrifugal pump. As the proportion of free gas within the fluid increases the gas tends to accumulate in the hub of the pump impellers, thereby causing the pump to lose prime, a condition known as “gas locking”. Accordingly, centrifugal pumps may not be entirely suitable for use in pumping mixed-phase fluids.
Other known pump types include plunger type positive displacement pumps and progressing cavity pumps.
Plunger type pumps are similarly affected by free gas entrained in the pumped fluid. In this case, the gas and liquid may separate within the pump barrel, which can cause a shock loading when the plunger descends and contacts the liquid surface, a condition known as “fluid pound”.
Progressing cavity pumps typically work by rotating a metal helical rotor within an elastomeric stator, the action of which causes discrete volumetric cavities to progress from the pump intake to the discharge. Although the mode of operation of such a pump makes it suitable for pumping liquids and gas, in practice gas tends to diffuse into the matrix of the elastomeric stator causing it to both swell and soften. As a consequence, the rotor may tend to either tear the stator and/or overheat due to decreased running tolerances and increased friction.
It is known that twin screw positive displacement pumps may reliably be used to produce multiphase fluids. The methods of constructing a twin screw pump and the essential elements of such a pump are well known to the person skilled in the art.
Typically, a twin screw pump may contain a single pair of intermeshing rotors, having oppositely handed screw threads and which rotate, in use, in opposite directions. The thrust generated by the pair of rotors as fluid is pumped through the pump may be borne by a suitable thrust bearing. Alternatively or additionally, a twin screw pump may be thrust balanced, i.e. it comprises two opposing pairs of intermeshing rotors, whereby the thrust generated by one pair of rotors is balanced by the equal and opposite thrust of the opposing rotor pair.
Regardless of the configuration of the pump, the screws of each pair of rotors must be synchronously rotated, typically by gearing the shaft of one rotor to the parallel shaft of the other such that the faces of the intermeshing rotors maintain a close clearance without clashing. Typically, some axial shaft adjustment means may be desirable to simplify the alignment of the start of the rotor threads with respect to each other.
Relatively simple screw mechanisms have been utilised for adjusting shaft alignment in twin screw pumps intended for use on the surface. Such mechanisms, however, are completely unsuitable for downhole or seabed pumps, since these pumps are typically extremely difficult to access for maintenance. Hence, it is much preferred that the rotors and shafts of a twin-screw pump for downhole use are aligned and fixed when the pump is assembled so that no further adjustment will be required during the service life of the pump.
In the past, most twin screw pumps have been produced having only a small number (typically only one) of pump stages; hence, they have often generally been unable to produce the extremely high differential pressures that may be necessary for lifting fluids within hydrocarbon wells.
In more recent times, some multistage twin screw pumps have been developed.
U.S. Pat. No. 5,779,451 discloses a pump which includes a housing having an internal rotor enclosure, the enclosure having an inlet and an outlet and a plurality of rotors operably contained in the enclosure. Each rotor has a shaft and a plurality of outwardly extending threads affixed thereon, the rotors being shaped to provide a non-uniform volumetric delivery rate along the length of each rotor. In one embodiment, the rotors have a plurality of threaded pumping stages separated by unthreaded non-pumping chambers. Although a multistage pump, the housing design precludes it from being used submerged within a well.
U.S. Pat. No. 6,413,065B1 discloses a modular multistage twin screw pump and a method of constructing the same. The stages may be selectively connected either in parallel or series, or any combination of the two, to produce the desired combination of pump pressure and flow rate. The pumps disclosed in U.S. Pat. No. 5,779,451 & U.S. Pat. No. 6,413,065B1 are thrust balanced.
Although suitable for use in a well, each individual module of the pump disclosed in U.S. Pat. No. 6,413,065B1 is extremely complicated containing as it does two shafts, two opposed pairs of intertwined and counter rotating rotors, an intake and discharge plenum and various fluid passages required to enable the individual pump stages to be hydraulically connected together either in series or parallel.
Moreover, a pump according to U.S. Pat. No. 6,413,065B1 would be extremely difficult to construct rapidly and/or in large volumes, not least because of the large number of discrete components which must be accurately aligned as the assembly is built, in particular the pairs of intertwined and counter-rotating rotors which must be axially secured to a common shaft to both control rotor end float (to prevent the screws clashing in operation) and transfer the rotor thrust to the common shaft to balance the opposing rotor thrust. In order to assemble such a pump, the shaft must first be passed through the central support (needle roller) bearing and the opposing rotors keyed, splined or otherwise rotationally secured onto the common shaft to transfer the drive from the shaft to the rotor. The fact that the rotors must be both axially and rotationally fixed to the shafts means that the manufacturing tolerances must be accurately controlled or complex shimming procedures must be used when assembling the pump to ensure the rotors are accurately aligned.
Further, as well as the rotating section, each module contains intake and discharge passages requiring the pump to have numerous different cross sectional profiles, further increasing the complexity of manufacture. In addition, each assembled module is secured with through bolts that necessitate a bulkhead between adjacent modules to provide access to torque them up.
The slow and complex manufacture and assembly of this pump means that it cannot readily be produced in sufficiently large numbers for large-scale commercial projects.
WO 03/029610 discloses another multi-phase twin screw pump for use in wells as well as a method of adapting a multi-phase twin screw pump for use in wells. The pump includes a housing having an intake end and an output end and a fluid flow passage extending between the intake end and the output end. Twin pumping screws are disposed in the fluid flow passage. A supplementary liquid channel extends through the housing in fluid communication with the twin pumping screws and a liquid trap is provided that is in communication with the fluid flow passage. In this way, liquid moving along the fluid flow passage by the pumping screws can be captured and fed through the supplementary liquid channel and returned to the fluid flow passage to enhance a liquid seal around the pumping screws.
However, the pump assembly taught in WO 03/029610 suffers many of the problems discussed above. In particular, assembly of the pump is very time consuming. The components must be assembled sequentially, each being accurately aligned with respect to adjacent components. This not only inhibits large scale production of this pump but makes “on the spot” maintenance of the pump extremely complicated and time consuming should the pump develop an operational problem
WO 95/30090 discloses an installation for pumping up liquids from the earth's crust, comprising: a screw pump lowered into the earth which is provided with a first screw member and a counter-screw member, drive means arranged on or close to the earth's surface for driving the screw member which in turn drives the counter-screw member; and transmission means for transmitting the drive produced by the drive means, which transmission means extend from the drive means on or close to the earth's surface to the lowered screw pump.
Further pump assemblies are described in RU 55050U1, WO 99/27256, GB2152587, GB 2376250 and EP 0464340, though none of these address the above-mentioned problems.
Hence, it is a non-exclusive object of the present invention to provide an improved multistage pump, which may in particular be quicker and simpler to assemble and/or more reliable and/or adaptable than known multistage pumps.